Expressions CS2: Data Structures and Algorithms Colorado State - PDF document

10/11/2017 Supplemental Materials: Grammars, Parsing, and Expressions CS2: Data Structures and Algorithms Colorado State University Original slides by Chris Wilcox, Updated by Russ Wakefield and Wim Bohm Topics Grammars Production Rules Prefix, Postfix, and Infix Tokenizing and Parsing Expression Trees and Conversion Expression Evaluation CS165: Data Structures and Algorithms – 2 Spring Semester 2017 Grammars Programming languages are defined using grammars with specific properties. Grammars define programming languages using a set of symbols and production rules. Grammars simplify the interpretation of programs by compilers and other tools. Grammars avoid the ambiguities associated with natural languages. CS165: Data Structures and Algorithms – Spring Semester 2017 3 1

10/11/2017 Definitions Grammar : the system and structure of a language. Syntax : A set of rules for arranging and combining language elements (form): – For example, the syntax of an assignment statement is variable = expression; Semantics : The meaning of the language elements and constructs (function): – The semantics of an assignment statement is evaluate the expression and store the result in the variable. CS165: Data Structures and Algorithms – 4 Spring Semester 2017 Ambiguity Natural Language: “British left waffles on Falklands.” Did the British leave waffles behind, or is there waffling by the British political left wing? “Brave men run in my family.” Do the brave men in his family run, or are there many brave men in his ancestry? CS165: Data Structures and Algorithms – 5 Spring Semester 2017 Language and Grammar A language is a set of sentences: strings of terminals -the words while, (, x < …. Grammar defines these, using productions LHS ::= RHS Read this as the LHS is defined by RHS CS165: Data Structures and Algorithms – Spring Semester 2017 6 2

10/11/2017 Language and Grammar LHS ::= RHS RHS is a string of terminals and non- terminals - Terminals are the words of the language – Non-terminals are concepts in the language – Non-terminals include java statements A sequence of productions creates a sentence when no non-terminal is left CS165: Data Structures and Algorithms – 7 Spring Semester 2017 Production Rules (Example) Non-terminals produce strings of terminals. For example, non-terminal S produces certain valid strings of a’s and b’s. – S ::= a S b – S ::= ba Valid: ba, abab, aababb, aaababbb, ... or a n bab n | n ≥ 0) Invalid: a, b, ab, abb, aba, bab, ... and everything else! CS165: Data Structures and Algorithms – 8 Spring Semester 2017 Example productions S ::= a S b or S ::= ba S  ba S  aSb  abab S  aSb  aaSbb  aababb S  a n bab n | n ≥ 0 CS165: Data Structures and Algorithms – Spring Semester 2017 9 3

10/11/2017 Production Rules and Symbols ::= means equivalence, is defined by < symbol > means needs further expansion Concatenation – x y denotes x followed by y Choice – x | y | z means one of x or y or z Repitition – * means 0 or more occurences – + means 1 or more occurences Block Structure : recursive definition – A statement can have statements inside it CS165: Data Structures and Algorithms – 10 Spring Semester 2017 Production Rules (Java Identifiers) <identifier> ::= <initial> (<initial> | <digits>)* <initial> ::= <letter> | _ | $ <letter> ::= a | b | c | ... z | A | B | C | ... Z <digit> ::= 0 | 1 | 2 | ... 9 Valid: myInt0, _myChar1, $myFloat2, _$_, _12345, ... Invalid: 123456, 123myIdent, %Hello, my-Integer, ... CS165: Data Structures and Algorithms – 11 Spring Semester 2017 Production Rules (Other Java) <Statement> ::= <Assignment> | <ForStatement > | … <ForStatement> ::= for (<ForInit> ; <Expression> ; <ForUpdate>) <Statement> <Assignment> ::= <LeftHand> <AssignmentOp> <Expression> <AssignmentOp> ::= = | *= | /= | %= | += ……. CS165: Data Structures and Algorithms – Spring Semester 2017 12 4

10/11/2017 Regular Expressions An alternative definition mechanism – Simpler because non-recursive Syntax used to define strings, for example by the Linux ‘grep’ command. Many other usages, for example Java String split and many other methods accept them. Two ways to interpret, 1) as a pattern matcher, or 2) as a specification of a syntax. CS165: Data Structures and Algorithms – 13 Spring Semester 2017 Regex Cheatsheet (1) Symbol Meaning Example * Match zero, one or more of previous Ah* matches "A", "Ah", "Ahhhhh" ? Match zero or one of previous Ah? matches "A" or "Ah" + Match one or more of previous Ah+ matches "Ah", "Ahh" not "A" \ Used to escape a special character Hungry\? matches "Hungry?" . Wildcard, matches any character do.* matches "dog", "door", "dot" [a-zA-Z] matches ASCII a-z or A-Z [ ] Matches a range of characters [^0-9] matches any except 0-9. CS165: Data Structures and Algorithms – 14 Spring Semester 2017 Regex Cheatsheet (2) Symbol Meaning Example Matches previous or next (Mon)|(Tues)day matches "Monday" or | character or group "Tuesday" Matches a specified number [0-9]{3} matches "315" but not "31" { } of occurrences of previous [0-9]{2,4} matches "12", "123", and "1234" ^http matches strings that begin with http, ^ Matches beginning of a string. such as a url. $ Matches the end of a string. ing$ matches "exciting" but not "ingenious" CS165: Data Structures and Algorithms – Spring Semester 2017 15 5

10/11/2017 Regex Examples (1) [0-9a-f] + matches hexadecimal, e.g. ab, 1234, cdef, a0f6, 66cd, ffff, 456affff. [0-9a-zA-Z] matches alphanumeric strings with a mixture of digits and letters [0-9] {3} -[0-9] {2} -[0-9] {4} matches social security numbers, e.g. 166-11-4433 [a-z] + @([a-z] + \.) + (edu|com) matches emails, e.g. whoever@gmail.com CS165: Data Structures and Algorithms – 16 Spring Semester 2017 Regex Examples (2) b[aeiou] + t matches bat, bet, but, and also boot, beet, beat,etc. [$_A-Za-z][$_A-Za-z0-9] * matches Java identifiers, e.g. x, myInteger0, _ident, a01 [A-Z][a-z] * matches any capitalized word, i.e. a capital followed by lowercase letters .u.u.u. uses the wildcard to match any letter, e.g. cumulus CS165: Data Structures and Algorithms – 17 Spring Semester 2017 Infix Expressions Infix notation places each operator between two operands for binary operators: A * x * x + B * x + C; // quadratic equation This is the customary way we write math formulas in programming languages. However, we need to specify an order of evaluation in order to get the correct answer. CS165: Data Structures and Algorithms – Spring Semester 2017 18 6

10/11/2017 Evaluation Order The evaluation order you may have learned in math class is named PEMDAS: parentheses → exponents → multiplication → division → addition → subtraction Also need to account for unary, logical and relational operators, pre/post increment, etc. Java has a similar but not identical order of evaluation, as shown on the next slide. CS165: Data Structures and Algorithms – 19 Spring Semester 2017 Reminder: Java Precedence parentheses ( ) unary ++ -- + - ~ ! multiplicative * / % additive + - shift << >> relational < > <= >= instanceof equality == != bitwise AND & bitwise exclusive OR ^ bitwise inclusive OR | logical AND && logical OR || ternary ? : assignment = += -= *= /= %= &= ^= |= <<= >>= >>>= CS165: Data Structures and Algorithms – 20 Spring Semester 2017 Associativity Operators with same precedence: * / and + - are evaluated left to right: 2-3-4 = (2-3)-4 CS165: Data Structures and Algorithms – Spring Semester 2017 21 7

10/11/2017 Infix Example How a Java infix expression is evaluated, parentheses added to show association. z = (y * (6 / x) + (w * 4 / v)) – 2; z = (y * (6 / x) + (w * 4 / v)) – 2; // parentheses z = (y * (6 / x)) + (w * 4 / v) – 2; // multiplication (L-R) z = (y * (6 / x)) + ((w * 4) / v) – 2; // multiplication (L-R) z = (y * (6 / x)) + ((w * 4) / v) – 2; // division (L-R) z = ((y * (6 / x)) + ((w * 4) / v))) – 2; // addition (L-R) z = ((y * (6 / x)) + ((w * 4) / v))) – 2; // subtraction (L-R) z = ((y * (6 / x)) + ((w * 4) / v))) – 2; // assignment CS165: Data Structures and Algorithms – 22 Spring Semester 2017 Postfix Expressions Postfix notation places the operator after two operands for binary operators: A * x * x + B * x + C // infix version A x * x * B x * + C + // postfix version Also called reverse polish notation, just like a vintage Hewlett-Packard calculator! No need for parentheses, because the evaluation order is unambiguous. CS165: Data Structures and Algorithms – 23 Spring Semester 2017 Postfix Example Evaluating the same expression as postfix, must search left to right for operators: (y * (6 / x) + (w * 4 / v)) – 2 // original infix y 6 x / * w 4 * v / + 2 - // postfix translation (y (6 x /) *) w 4 * v / + 2 - ((y (6 x /) *) w 4 * v / + 2 - (y (6 x /) *) (w 4 *) v / + 2 - (y (6 x /) *) ((w 4 *) v /) + 2 – ((y (6 x /) *) ((w 4 *) v /) +) 2 - (((y (6 x /) *) ((w 4 *) v /) +) 2 -) CS165: Data Structures and Algorithms – Spring Semester 2017 24 8